1. Field of the Invention
The present invention generally relates to thermal heads and, more particularly, is directed to a thermal head which is used as a printer for computers, personal computers and so on or as a recording means for a as facsimile equipment or the like.
2. Description of the Prior Art
Recently, as the performance of computers, personal computers and so on are enhanced more and more, it is desired that the performance of a printer which serves as a recording apparatus therefor is also enhanced, which naturally requires high speed and high density printing.
As a recording system of such a printer, a conductive thermal printing system is known. According to this conductive thermal printing system, the printing is carried out by directly heating an ink layer of an ink ribbon or the like by the conduction of the ink ribbon by means of, for example, conductive electrodes. This printing system has an excellent in the printing speed and for this reason, the development of the conductive thermal printing system is remarkable.
Such a conductive thermal printing system will be described with reference to FIG. 1. FIG. 1 illustrates a perspective view of a main portion of an example of a conventional thermal head.
In FIG. 1, reference numeral 21 designates an insulating substrate made of, for example, ceramics. A conductive thin film made of aluminum (Al) or the like is deposited on the entire surface of a major surface 21A thereof by some suitable process such as a vapor deposition, a sputtering process, a screen printing process or the like. Then a pattern etching is performed, for example, a patterning is performed in a range of from an end face 21B to an end face 21C of, for example, an insulating substrate 21 in a straight line fashion to thereby form a wiring pattern 24. This wiring pattern 24 serves as a conductive thermal electrode 23 to construct a thermal head 10.
The conductive thermal electrode 23 formed by the above-mentioned method has a cross section such that an aspect ratio thereof i.e. a ratio of the height of the wiring pattern 24 relative to its width is less than 1. For this reason, in printing, the conductive thermal electrode 23 is inclined so as to make the aspect ratio close to 1. FIG. 2 is a schematic cross-sectional side view illustrating the recording condition of the conventional thermal head 10.
As shown in FIG. 2, if the conductive thermal electrode 23 is conducted under the condition such that one end face of the conductive thermal electrode 23 i.e. a conductive thermal electrode end 23A is obliquely brought in contact, for example, with an ink ribbon 28, the ink layer of the ink ribbon 28 is heated and melted at its portion where the ink ribbon 28 is brought in contact with the conductive thermal electrode end 23A. The thus heated and melted ink layer is exuded onto a printing paper urged against the ink ribbon 28 by a platen or the like, the printing thus being made.
In the above printing method, however, in order to prevent the area in which the insulating substrate 21 contacts with the ink ribbon 28 from increasing because the thermal head 10 is worn, the insulating substrate 21 must be cut-away as shown by a broken line in FIG. 2, which makes the configuration of the thermal head 10 complicated. Such a complicated thermal head 10 cannot be produced efficiently.
The wiring pattern 24 of high density must have a width of, for example, 60 .mu.m, a height of 60 .mu.m and a pitch of 125 .mu.m in order to obtain a printing of high density and high resolution, both of which are recent demands. Such a high density wiring pattern cannot be made without difficulty, and lead wires cannot be led out without difficulty from each of the conductive thermal electrodes 23 formed of the high density wiring pattern 24, which hinders the thermal head from being produced efficiently. Furthermore, defective wiring brings about an inferior thermal head, which unavoidably lowers productivity.
To solve the above-mentioned problems, such a thermal head structure is proposed, in which lead wires, serving as conductive thermal electrodes, are embedded in grooves formed on a substrate by a mechanical cutting process, a laser machining process or the like. However, the mechanical machining process of high density is difficult to perform, and this thermal head structure causes the number of assembly processes to increase, which as a result hinders the thermal head from being produced efficiently.
Furthermore, such a proposal for the thermal head structure is also made, in which a flexible printed circuit board (i.e. FPC) is used and conductive portions interconnected within this FPC are used as conductive thermal electrodes without modifications thereof. However, because the base material of the FPC has a poor wearproof property, the thermal head characteristic is deteriorated. Also, since the conductive portions constructing the electrodes are made by printing techniques, such as a printing process or the pattern etching process of metal thin film or the like, the aspect ratio of the cross section of the conductive portion i.e. electrode becomes comparatively small. From this standpoint, it is difficult to obtain a printing of high density and high resolution.
A further proposal provides such a thermal head structure such that parallel flat wires in which fine conductive wires, each having a diameter of about 60 to 80 .mu.m, are aligned in the electrically isolated condition within a polymer resin. These are used as a conductive thermal electrode to form a thermal head. In this case, in order to hold the flexible parallel flat wires without being displaced on a printing surface, the assembly work thereof becomes complicated.